I (think I) have just finished upgrading my system to OS X 10.5. I'm hesitant to declare it a success because I haven't tried everything yet, though I've been reading e-mail and doing most 'normal' things. Apple made it difficult, but not impossible, thank goodness.
According to the documentation, all you do to move from 10.4 to 10.5 is an 'upgrade.' Perhaps this is true for someone, but evidently not for foolish people like myself who value reliability enough to RAID the system volume
I had already resigned myself to not RAIDing 10.5. I had un-RAID-ed my system volume from Disk Utility - something you have to do from the distribution media, since of course you can't un-RAID a volume your system is running on at the moment.
That wasn't enough. 10.5 refused to acknowledge that my volume could be updated. It has to be wiped.
I tried running Disk Util's First Aid function and, sure enough, it fixed a bunch of stuff. But that wasn't enough, either.
Finally I bit the bullet and erased the volume, installing 10.5 atop a clean file system.
I took the mirror from my un-RAID-ed pair, and used it as the 'previous system' for the Migration Assistant. It took a while to move 'everything' when it's about 100 gig.
broke, and it made the second or third reboot a little sketchy. The system hung itself on something, I suspected PGP, and I tried booting in single user mode (typing S or something while rebooting). I didn't land in single user mode, but things rebooted cleanly.
I uninstalled PGP and found out that my purchase last July doesn't qualify me for an upgrade from 9.6 to 9.8 (tho' 9.6 doesn't run on the latest OS X).
is finally(!) available for the Mac, so I suppose it's time to give it a try.